1. Field of the Invention
The present invention relates to an underlayer for use in a high density magnetic recording media. More particularly, the invention relates to an underlayer for use in a high density magnetic recording media comprising A1Pd or CoTi intermetallic compound of B2 crystal structure, or Co.sub.50 Ti.sub.50-x M.sub.x metal alloy in which Ti in CoTi intermetallic compound is partly substituted by other substitutional elements while maintaining its B2 crystal structure, or CoTi/Cr having a double thin film structure in which a Cr seed layer is introduced. The present underlayer has a crystal structure and microstructure suitable for a high density magnetic recording media so that it provides a good texture structure with a CoCr-based magnetic layer deposited thereon and shows fine grain size distribution, high coercity and high coercity squareness.
2. Description of the Prior Art
An information recording media of a computer hard disk drive uses a magnetic recording process and refers to a magnetic disk. The magnetic disk generally comprises a substrate, underlayer, magnetic layer and protective coating layer (FIG. 1). The magnetic layer uses a Co-based alloy thin film of HCP structure having high crystallographic anisotropy, and the magnetic property of its longitudinal direction can be improved by enhancing the crystallographic orientation property of the c-axis which is a magnetic easy axis or by fining a grain size of a thin film.
An underlayer deposited between the substrate and the magnetic layer increases an adherence of the magnetic thin film and acts as a template on the magnetic layer deposited thereon by which it makes possible to control the microstructure of the magnetic layer. An important role of the underlayer as the template is to grow its crystal surface in a preferred orientation so as to have a grain-to-grain epitaxial growth relationship with the crystal surface of the magnetic easy axis of the magnetic layer. To meet this requirements, many different underlayer materials such as Cr, Mo, W, Ti, NiP and Cr alloy with the other elements, for example CrV, CrTi, CrW, etc., have been researched (See, H. Susuki, N. Tsumita, M. Hayashi, Y. Matsuda, IEEE Trans. Magn., 26, 2280 (1990) and Rajiv Ranjan, J. Appl. Phys., 67 (9), 4698 (1990)).
Among these, the most successful underlayer material to be used is a Cr thin film having BCC structure which can grow heteroepitaxially with Co alloy magnetic thin film. The crystallographic orientation and grain size of the Cr underlayer is important for the control of those of Co alloys. However, grain size of Cr is relatively large unless it is deposited in ultra high vacuum, and strict crystallographic texture control is sometimes difficult.
Recently, a NiAl intermetallic compound underlayer having B2 crystal structure which has the similar lattice constant as that of Cr (2.884 .ANG.) and can control the fine grain size has been proposed as an alternative for the Cr underlayer (See, L. L. Lee, D. E. Laughlin, and D. N. Lambeth, J. Appln. Phys., 79(8), 4902 (1996)). However, although the NiAl underlayer material can make the grain size finer because of the high binding energy property of an intermetallic compound, a higher temperature and a thicker thin film is required to form a crystalline thin film since the temperature necessary for forming its stable phase is high, about 1635.degree. C. Also, when NiAl thin film is deposited directly on an amorphous substrate, only the NiAl (110) crystal plane grows so that it is impossible to make the magnetic easy axis of Co alloy magnetic thin film coated thereon grow in parallel on the surface of the substrate. To improve this, a seed layer such as MgO should be introduced into the NiAl underlayer. Therefore, the NiAl intermetallic compound has many factors to be improved in view of heteroepitaxial growth.
For this reason, skilled persons in the art have carried out research to develop a novel underlayer to supplement the above disadvantage and to obtain a magnetic layer having the magnetic property and microstructure suitable for a high density magnetic recording media.
Considering the problems derived from the above conventional underlayer material for a high density magnetic recording media, the present inventors investigated whether an A1Pd or CoTi intermetallic compound having B2 crystal structure with a lattice constant of 2.987 .ANG. which is expanded about 3.8% in comparison to a Cr underlayer having BCC structure, which is currently used as an underlayer of CoCrPt(Ta) magnetic thin film for the magnetic recording media, or a NiAl having B2 crystal structure proposed as an alternative for the Cr underlayer, or Co.sub.50 Ti.sub.50-x M.sub.x metallic alloy in which Ti in CoTi intermetallic compound is partly substituted by other substitutional elements while maintaining its B2 crystal structure, can be used as underlayer material for a high density magnetic recording media. Further, they researched whether introduction of a Cr seed layer to the CoTi underlayer can improve crystallization of the underlayer.
From this research, the present inventors found that when said AlPd or CoTi intermetallic compound having B2 crystal structure, Co.sub.50 Ti.sub.50-x M.sub.x metal alloy, or CoTi/Cr underlayer of a double thin film structure in which CoTi is coated on Cr seed layer is used as underlayer material, the underlayer provides a magnetic recording media having superior magnetic property and microstructure, makes a good texture structure with Co-based magnetic layer deposited thereon and shows fine grain size distribution, high coercity and high coercity squareness.